A styrenic resin is excellent in mold processability and mechanical properties, therefore, the resin has been widely applied to electric appliances. However the styrenic resin is extremely easily flammable, which can easily make a fire accident. In this regard, the resin is subjected to various mandatory controls on their flammability for the safety reason in United States, Japan and Europe and is required to have high flame retardancy to meet the Underwriter's Laboratories Standard for the use of the housing of electric appliances.
A widely known method for flame retardancy is that a halogen-containing compound and an antimony-containing compound are added to a rubber modified styrene-containing resin to give a good flame-retardant property. The examples of the halogen-containing compounds used in the method above are, for example, polybromodiphenyl ether, tetrabromobisphenol-A, epoxy compounds substituted by bromine, chlorinated polyethylene, etc. An antimony trioxide or an antimony pentaoxide is commonly used as an antimony-containing compound.
The methods for improving the flame-retardant property by applying a halogen- and antimony-containing compound have advantages such as easy acquirement of the flame-retardant property and no degradation of the physical properties. However, the disadvantages could be observed that the halogen-containing compound results in the corrosion of the mold itself by the hydrogen halide gases released during the molding process and is fatally harmful due to the toxic gases liberated in case of fire. Especially, since a polybromodiphenyl ether, mainly used for a halogen-containing flame retardant, can produce toxic gases such as dioxin or furan during combustion, a major concern in this field is to develop a flame retardant which is prepared without a halogen-containing compound.
U.S. Pat. No. 4,526,917 discloses use of a TPP and a mono phosphoric acid ester compound having a mesityl group to a base resin comprising a polyphenylene ether resin and a styrenic resin to improve flame retardancy of the resin composition. However, in the patent, the amount of polyphenylene ether is more than 20%, which makes it difficult to obtain dripping flame retardancy while maintaining its flowability. Moreover, the heat resistance of the resin composition is lowered due to addition of TPP.
Japanese Patent Application No. 7-043,769 discloses that the use of phosphoric acid ester compound having a substituent containing 12 to 25 carbon atoms derived from TPP may obtain flame-retardancy of V2 and improved volatility at 250° C.
U.S. Pat. Nos. 3,789,091 and 3,849,368 disclose that a ring-type phosphonic acid ester compound can be used as a flame retardant to a polyethylene terephthalate, PAN, polyurethane, polystyrene or nylon.
In view of the above-mentioned patents, there have been many attempts to provide flame retardancy by using a phosphoric acid ester compound as a halogen-free flame retardant, but none of the publication discloses a resin composition with good mechanical strength, heat resistance and dripping flame-retardancy without polyphenylene ether by using only phosphorus-based compound and rubber modified polystyrene resin.
So, the present inventors have developed a flame retardant styrenic resin composition which has good flame retardancy as well as mechanical strength and heat resistance by employing a ring-shaped alkyl phosphonic acid ester compound as a flame retardant to a base resin consisting of rubber modified polystyrene resin. The resin composition of the present invention may further contain polyphenylene ether optionally.